The present invention relates to a direction adjustment tool for a downhole drilling apparatus incorporating the tool, and relates particularly to a tool for correcting the direction of drilling of the drilling apparatus when it deviates from vertical. The invention relates particularly, but not exclusively, to such a tool for use in oil and gas well drilling apparatus.
Drilling direction adjustment tools are known for correcting the direction of drilling of a drilling apparatus incorporating the tool, and which include devices such as accelerometers or magnetometers, which provide an electrical signal representing the deviation of a longitudinal axis of the tool from the vertical. Electrical signals representing deviation of the longitudinal axis of the tool from the vertical are used to control the direction of drilling of the apparatus, for example by means of steering pushers which engage a wall of a borehole formed by the drilling apparatus, to cause the orientation of the tool to deviate, which in turn adjusts the direction of drilling back towards the vertical.
Existing tools of this type suffer from the drawback that the use of complicated electronic components increases the cost of production of the apparatus, and makes the apparatus more prone to failure. This can be particularly disadvantageous when the tool is located downhole, since drilling operations must be ceased while the tool is recovered.